1. Field of the Invention
The present invention relates to reinforcements for concrete, and in particular, the use of E-glass fibers for reducing plastic shrinkage cracks in concrete.
2. Technical Considerations
Concrete is a well known and widely used construction material.
As concrete is fundamentally weak in tension and has low ductility, fibers have been used to reinforce concrete. A primary function of the fibers is to arrest advancing cracks and delay their propagation across the concrete matrix by distributing the shrinkage stresses. For example, due to high shrinkage during initial curing and setting of concrete (i.e. the first 24 hours), concrete tends to develop fine, hairline shrinkage cracks. These cracks enlarge over time and permit carbonation, and allow corrosive salts to penetrate and corrode the concrete and steel reinforcing. In addition, freeze-thaw cycles further accelerate the growth of these cracks and the accompanying deterioration of the concrete. As a result of the reduction in plastic shrinkage cracks, the long-term durability of the concrete is improved.
To prevent the formation of these shrinkage cracks, synthetic fibers, and preferably polypropylene and nylon fibers, are typically added to the concrete during the mixing stage. However, due to the difference in densities between the concrete ingredients and the polymeric fibers, the polymeric fibers have a tendency to segregate themselves from the other materials in the concrete mix and float to the upper portions of the setting concrete. This segregation of the polymeric fibers in the concrete reduces the overall effectiveness of the fibers in reducing shrinkage cracks as well as lower its aesthetic value. In the concrete industry, this condition is sometimes referred to as xe2x80x9cfuzzy concretexe2x80x9d. In addition, the adhesion of polymeric fibers to the cement in concrete is poor, thus reducing the effectiveness of the fibers in preventing shrinkage cracks from forming.
The effectiveness of glass fibers as reinforcements is reduced because glass fibers are generally brittle and therefore are prone to damage or strand separation by impact and abrasion during conventional concrete mixing processes. In addition, E-glass fibers are not used to reinforce concrete because the glass fibers deteriorate in the alkaline environment of the cement. This characteristic of E-glass resulted in the development of alkaline resistant glass fibers and coatings.
U.S. Pat. No. 5,298,071 discloses a fiber-hydratable cement composition made by combining fibers with cement clinkers in a grinding mill and grinding the mixture into a fine powder. The fibers can be alkaline resistant glass, ceramic, carbon, or organic polymer, and preferably steel, polyolefin or mixtures thereof.
U.S. Pat. No. 5,628,822 discloses a concrete reinforce with synthetic fibers having a variety of different fiber designs comprising variations in length, width, thickness, denier, fibrillation and aspect ratios. The fiber is preferably made of polypropylene.
U.S. Pat. No. 4,935,301 discloses a cement composite containing bundles of glass fibers encapsulated, rather than impregnated, with a polymeric coating.
U.S. Pat. No. 5,643,359 discloses reinforcing concrete with pulp fiber from wood and non-wood plants or recycled paper products. The fibers are about 0.1 to 30 mm long and about 0.001-0.1 mm in diameters.
JP 6[1994]-87,326 discloses coating glass fibers with a mixture of silicone and thermoplastic resins to provide alkaline resistant glass fibers.
U.S. Pat. Nos. 4,008,094; 4,036,654 and 5,064,785 discloses glass compositions that include zirconium to provide the glass with high alkaline resistance. U.S. Pat. No. 4,882,302 discloses alkaline resistant glasses that include at least one lanthanide series metal oxide.
It would be advantageous to provide a fiber material that is less expensive than specially formulated alkaline resistant glass compositions or other synthetic fibers and is effective in reducing plastic shrinkage cracks in concrete.
The present invention provides concrete having improved plastic shrinkage crack resistance, comprising: concrete; and non-alkaline resistant glass fibers. In one nonlimiting embodiment of the invention, the non-alkaline resistant glass fibers are E-glass fibers that comprise at least 0.025 percent by volume of the concrete, and the fibers have lengths ranging from 1.27 cm to 5.08 cm and diameters ranging from 5 micrometer to 23 micrometers.
The present invention also provides a method of reducing plastic shrinkage cracks in concrete, comprising the steps of: combining cement, aggregate, sand, and water to form concrete; adding a desired amount of non-alkaline resistant glass fibers to the concrete; and mixing the concrete and glass fibers to disperse the glass fibers uniformly throughout the concrete, wherein the glass fibers reduce the amount of plastic shrinkage cracks of the concrete as compared to non-fiber reinforced concrete. In one nonlimiting embodiment of the invention, the fibers are E-glass fibers that comprise at least 0.025 percent by volume of the concrete, and the fibers have lengths ranging from 1.27 cm to 5.08 cm and diameters ranging from 5 micrometer to 23 micrometers.